Lewis Acid/Base Interaction Matrix Database
van der Waals Complexation
|Lewis Base–Proton–Lewis Base Complex|
There are several types of van der Waals attraction:
It is tempting to consider these forces to be of different strengths, but it is the distance range that is more important. Dipole/dipole attraction is relatively long range in action while the London spontaneous-dipole/Induced-dipole attraction requires contact between the van der Waals surfaces: the molecules need to touch.
Molecules with permanent dipole moments, polar molecules, such as iodine chloride, ICl, exhibit dipole-dipole attraction. The iodine end of iodine chloride is δ+ and the chlorine end is δ–. Molecules interact with each other so that the dipoles line up end-to-end:
All molecules with a permanent dipole exhibit permanent-dipole/permanent-dipole attraction. At temperatures below the material’s melting point, the structure will show long range order and crystallinity.
Permanent-Dipole/Induced-Dipole Attraction :
Molecular dipoles (polar molecules) are able to induce weak dipoles in adjacent non-polar species. The effect gives rise to a weaker attraction than dipole-dipole attraction.
London Dispersion Force (LDF) Attraction:
The very fact that it is possible to liquefy helium – and indeed all molecular materials – demonstrates that there must be some type of inter-molecular attraction taking place between the helium atoms. (Helium is a molecular material, where the helium molecule consists of just one atom.)
The attraction is known as the London dispersion force and is deemed to arise from short time scale fluctuations in the electronic structure of species which results in the formation of instantaneous dipoles.
The instantaneous-induced-dipole/induced-dipole London dispersion forces (LDF) are surprisingly strong but they only act at very short range. It is as if the surface of even neutral, non-polar molecules like methane, CH4, are 'sticky'.
All molecules exhibit London dispersion forces and the strength increases with the size/surface area of the molecule. This logic can be used to explains the increasing boiling and sublimation temperatures of the halogens.
Going down the periodic table the atoms become larger, so the diatomic molecules become larger and their surface area becomes larger. Thus, the van der Waals forces increase and so do the boiling points:
F2 < Cl2 < Br2 < I2
Likewise, longer chain alkanes have higher boiling points than shorter chain alkanes. Branching, which decreases surface area, reduces boiling point.
Which is stronger: dipole/dipole or London forces?
Gecko Toes, Setae and van der Waals Forces:
"The toes of the gecko have developed a special adaptation that allows them to adhere to most surfaces. Recent studies of the spatula tipped setae on gecko footpads demonstrate that the attractive forces that hold geckos to surfaces are van der Waals interactions between the finely divided setae and the surfaces themselves. Every square millimeter of a gecko's footpad contains about 14,000 hair-like setae." Wikipedia:
As a first approximation: "Like Dissolves Like":
|Congeneric Series:||The congeneric series concept is not useful here.|
|Molecular Shape Recognition Complex (generic)
|Enzyme/Substrate Complex (generic)
|Oxonium ion/18-Crown-6 Complex
|Potassium ion 18-crown-6 complex
|Zinc finger protein/DNA Complex (generic)
|Interactions and reactions classified as:|
|Poster||Nucleophiles & Bases|
© Mark R. Leach 1999 –
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